Purifying induction lamp target



United States Patent PURIFYING INDUCTION LAlVIP TARGET Laurie H. Sabine, Beverly, Mass., and Selwyn L. Willey,

Sunnyvale, Calif., assignors to Sylvania Electric Products Inc., Salem, Mass., a corporation of Massachusetts N0 Drawing. Application February 28, 1956 Serial No. 568,153

2 Claims. (Cl. 23-208) This invention relates to induction lamps, and particularly to methods of purifying the targets of such lamps.

The target can be a piece of tantalum carbide, for example, although other highly refractory materials, for example niobium carbide and hafnium carbide, can be used.

During the operation of the lamp, crystal growth occurs around the target, reducing the light output and eventually short-circuiting the space between the target and its surrounding elements, especially if a magnetic field concentrator is used around the target.

The impurities normally present in tantalum carbide and other carbides facilitate the growth of such crystals and are detrimental to the life of the lamp.

The commercially available tantalum carbide, for example, is made by sintering a mechanical mixture of tantalum and carbon, which is then ground to a powder. The lamp target is made by heating and pressing the raw tantalum carbide powder until it is sintered into a compact mass. The resultant material will have a density nearly as high as the theoretical value for the completely homogeneous substance.

However, we have found that the tantalum carbide target so produced contains not only the desired carbide of tantalum, TaC, but also contains metallic tantalum, a lower carbide of tantalum, Ta C, some carbon and small amounts of other impurities.

We find that these can be removed by heating the target in an atmosphere of carbon-containing gas, such as a hydrocarbon, at a temperature well above that at which the material was originally formed or has encountered in previous processing. The heating will vaporize the impurities, other than Ta C, and Ta, and the latter will be carbonized by the hydrocarbon gas to form TaC, the desired material.

The target can then be cleaned mechanically and used directly in a lamp, but we prefer to heat it further in an inert atmosphere or vacuum to drive off excess carbon which might cause undesirable blackening of the lamp bulb.

Targets prepared in the above have a longer useful life and can be operated at higher temperatures, due to the apparent increase in melting point.

Other objects, advantages and features of the invention will be apparent from the following description of a specific embodiment.

For example, a tantalum carbide target of cemmer- .cially-available material is heated inductively in a bell jar in an atmosphere of 50% methane and 50% of an inert diluent gas, such as argon, neon, helium or the like, at a pressure of about 1200 mm. of mercury. Although a specific pressure is given, the pressure can be varied widely with good results. The content of methane, or other hydrocarbon gas, can also be varied considerably, contents from about 25% to 100% being satisfactory, although we prefer 50%.

The target is brought to a temperature sufficient to produce a cloud of smoke in the jar. The smoke is the visible evidence of the reaction of the gas with the target material, that is of the carburization. The temperature 2,856,269 if'atentecl Oct. 14-, 1958 for this will be above 3000 K. but below the melting point of the carbide. The exact temperature is difficult to measure, because the smoke around the target shields it from such temperature-measuring devices at optical pyromoters, and the best indication that the temperature is above the proper point is the presence of smoke. The process is continued until the smoke clears up. That will ordinarily take about 10 minutes.

The target can then be used in a lamp, after cleaning mechanically, for example, by brushing or scraping off the deposit formed over the surface of the target, particularly on the top surface of the latter. We prefer, however, to subject the target to a further heating step before using it in a lamp. We accordingly again place the target in a bell jar, fill the latter with argon at a pressure of about 1200 mm. of mercury for example, and heat the target again inductively. This time the target is brought to a higher temperature, preferably in excess of 3400 K. but below the melting point of the carbide, for about 15 minutes.

The presence of the argon can vary widely for good results, although there would not ordinarily be any gain in using much more than 1200 mm. Actually, we have found that heating in a vacuum, without the presence of any appreciable amount of argon or other gas, is somewhat preferable. For example, a vacuum as low as 5 microns pressure has been used.

The latter heating step drives off any excess carbon which may remain on the target.

If not so removed, the carbon would cause excessive blackening of the lamp during operation, thus shortening its effective life.

The heating of the target can be achieved in the usual manner for induction heating, for example by placing a suitable coil around the target in the bell jar and sending high-frequency current through the coil.

Although the invention has been described in connection with a specific embodiment, it will be understood that the invention is not limited thereto, and that various modifications will be apparent to one skilled in the art without departing from the scope and spirit of the invention. For example, although we have specifically mentioned methane gas in the preferred embodiment other hydrocarbon gases such as ethane, ethylene, propane, and the like can be used.

Although in the embodiment described above the target is heated in methane only once, we prefer to pump the bell jar out after the first heating, and then refill the jar with methane and repeat the heating step as before.

What we claim is:

1. The process of purifying a metal carbide target which comprises: heating said target in an atmosphere of hydrocarbon at a temperature below its melting point but above the temperature at which smoke is evolved from said target, and thereafter heating said target at a temperature between 3400 K. and 3700 K. in an inert atmosphere at low pressure.

2. The process in which the target is subjected to the process of claim 1 in a bell jar, and is thereafter mounted in an electric lamp bulb.

References Cited in the file of this patent UNITED STATES PATENTS 1,909,680 Hunter May 16, 1933 FOREIGN PATENTS 573,564 Great Britain Nov. 27, 1945 OTHER REFERENCES Schwarzkhop et al.: Refractory Hard Metals, 1953, p. 112. 

1. THE PROCESS OF PURIFYING A METAL CARBIDE TARGET WHICH COMPRISES: HEATING SAID TARGET IN AN ATMOSPHERE OF HYDROCARBON AT A TEMPERATURE BELOW ITS MELTING POINT BUT ABOVE THE TEMPERATURE AT WHICH SMOKE IS EVOLVED FROM SAID TARGET, AND THEREAFTER HEATING SAID TARGET AT A TEMPERATURE BETWEEN 3400*K. AND 3700*K. IN AN INERT ATMOSPHERE AT LOW PRESSURE. 